The electrical power generating companies that operate in the United States and throughout the world utilize coal fired boilers to generate electrical energy. The coal is combusted in a burner and the heat used to generate high pressure steam. The steam drives a large turbine which in turn drives an electrical generator.
The basic concept for generating electrical power from combusting coal is straight forward. The design and implementation of the process is however complex. Many critical features must be met within the design, fabrication and operation of a coal fired power plant. One critical component is the coal combustion and heat exchanger unit. One design is referred to as a "cyclone boiler" within the power generation industry. This design has been implemented in over one hundred power plants throughout the world to utilize coal for the generation of electric power.
FIG. 1 shows the design and operation of a cyclone boiler 10. Combustion air 12 is blown into the side of the cyclone tangent to the cylindrical axis. Coal 11 is injected in a similar manner at one end of the burner. As the coal and combustion air 13 swirl in a cyclonic fashion through the burner, the coal 11 is combusted. Heat released from coal combustion heats the walls of the cyclone boiler 10 which in turn provides the ignition temperature for incoming coal 11 and combustion air 13. The result is a steady state burn of coal and air as they are mixed in the cyclone boiler 10. Multiple cyclone boilers are used on a power generating unit. The highest operating temperatures in the power generating unit are achieved inside the cyclone boiler 10.
The incoming coal 11 has some non-combustible constituents. These non-combustible constituents melt from the heat in the boiler and form slag. Much of the slag is deposited on the walls of the cyclone boiler 10 by the cyclonic action of the gases as they pass through the burner. Gravity pulls the slag down the walls and then out through a slag hole. The slag hole is at a low point in the exhaust end of the burner.
The radiant heat from coal combustion and the convective heat from the hot gases and slag are used to generate high pressure steam. To accomplish this the cyclone boiler walls are built from steel tubing called boiler tubes 14. Water is pumped through the boiler tubes 14 to collect the heat and to prevent the tubes from melting. The pressure and flow rate of water are controlled to generate the desired steam temperature and pressure.
If the boiler steel tubing directly contacts the slag and the hot combustion gases it results in rapid erosion of the tubing surface. To prevent this the tubes have a refractory coating applied over the surface which limits the tube exposure to the combustion gases and slag
During the service life of the boiler, the slag erodes the refractory coating leaving the boiler tubes exposed. The exposed tubes erode until the wall thickness of the boiler tubes make the cyclone boiler unusable. At the end of life it becomes necessary to replace the cyclone boilers 10. The boilers are cut out and new ones welded in. This is an expensive process and adds substantially to the cost of operating a power plant.
A method to repair these eroded boiler tubes 14 using weld overlay 18 is shown in FIG. 2. A boiler tube cross section 16 is illustrated before and after application of weld overly 18. Erosion of the boiler tubes 14 produces wall thinning 17 on the side of the tube exposed to the combusting coal and air. By application of weld overlay 18 to the boiler tube the wall thickness can be repaired. FIG. 2 illustrates a repaired boiler tube wall 19. Repair of the boiler tube wall can greatly extend the service life of a cyclone boiler 10 resulting in substantial savings in operating costs.
The difficulties in applying weld overlays 18 in a reliable manor are substantial. The areas of difficulty which must be solved by any method used are as follows:
Scheduled shutdown periods for maintenance on power plants are held to minimal time to maximize revenues. These scheduled shutdowns are too short in duration to utilize manual welding techniques. However, a fully automated method of weld overlay can meet the time requirements for scheduled plant shutdowns. Also the weld quality is more consistent with automated welding than with manual methods.
The boiler tubes 14 are distorted which results from years of use and repair work done in the boiler. This distortion creates an irregular path for the weld overlay. To automate the weld overlay process, each boiler tube must be measured to determine a weld path prior to welding. The automated system should perform both the measurement of the tubes and the weld overlay process. To make the weld path measurements, a frame of reference for measuring needs to be created in the cyclone boiler. The automated welding system must provide this frame of reference.
The boiler tubes 14 must be repaired in place. Access to the tubes is obtainable only through a 34 inch access man hole. Furthermore, there are no flat surfaces in the boiler to work from as equipment staging areas. The interior profile is a horizontal cylinder with a split wall at the top of the cylinder. The split wall has a radial offset creating a tangential opening for combustion air. The ends of the cyclone boiler are conical sections with the man hole access in one end. Several complex tube geometries exist in a cyclone boiler 10. To automate weld overlay on these tubes requires an innovative and versatile automated mechanism which can be installed in the cyclone boiler and manipulate a welding torch to track these various geometries. Heretofore, existing automated welding systems have not provided an easy to use, low labor means which meet these criteria that could be used to repair boiler tubes 14 in place in a cyclone boiler 10. This invention provides an innovative means for applying weld overlay 18 on cyclone boiler tubes. This invention called a Boiler Automated Welding System (BAWS) uses a unique and innovative combination of sensing technology, welding system, computer systems and a automated motion system. This system applies weld overlays 18 by a highly automated process which accommodates the cyclone boiler geometry and the varying wall thickness of boiler tubes 14.